Cyclic voltammetry was used to measure the rates of the chemical oxidation of guanine (G), guanosine (Gs), 2'-deoxyguanosine (dG), and 2'-deoxyguanosine Y-monophosphate (dGMP) by electrochemically generated tris(2,2'-bipyridyl)ruthenium(III). The numeric fit of voltammograms to an ECCCE type of mechanism provided the equilibrium and rate constants of the two-step chemical oxidation of the guanine species. One-electron redox potentials evaluated from the equilibrium constant of the first electron uptake follow the sequence G < Gs approximate to dG approximate to dGMP, indicating that guanine is oxidized most easily. This sequence is expressed in the rate constant, which apparently follows the expected driving force dependence. Ab initio molecular orbital calculations were carried out using the DFT/B3LYP method with 6-31G** and 6-31++G** basis sets, and also the RI-MP2 method with the cc-pVDZ basis set, so as to clarify the role of various factors contributing to the redox potential. Theoretical results suggest that the difference between the one-electron redox potentials of Gs and G (ca. 0.13 V) originates partly from the higher energy of proton dissociation from the cation radical Gs(.+) and partly from the higher difference in the hydration energy between the deprotonated radical Gs(-H)(.) and the parent Gs, which compensate for the lower ionization potential of Gs compared to that of G.